Assessment of the influence of inhibitors on carbon dioxide and methane emissions from soils irrigated with alternative water resources

The reduction of greenhouse gas releases from agricultural systems is of great significance in mitigating climate change. It is necessary to implement measures to mitigate carbon dioxide (CO2) and methane (CH4) emissions from soils irrigated using alternative waters, promoting the reuse of livestock wastewater and reclaimed water. The use of nitrapyrin, a nitrification inhibitor, and N-(N-butyl) thiophosphoric triamide, a urease inhibitor, has been explored to reduce greenhouse gas emissions. However, the impact of the inhibitor application on the soil carbon cycle as well as CO2 and CH4 emissions under alternative water irrigation remains unclear. To address this, a pot experiment with two consecutive cycles, irrigated with reclaimed water, livestock wastewater, and groundwater control was conducted. Nitrapyrin and/or N-(N-butyl) thiophosphoric triamide were applied to investigate the impacts of inhibitors on carbon cycle functional genes, soil properties, and CO2 and CH4 releases under alternative water irrigation. The results showed that inhibitor application increased the enrichment level of carbon degradation functional genes (xylA and cdh) and CO2 emission in the first cycle of this experiment, but had the opposite effect in the second cycle, regardless of the water type. The effects of increasing the relative abundance of methane-oxidizing genes (mmoX and mxaF) and lessening CH4 emissions were more pronounced when the nitrification inhibitor was applied alone. The combined application of inhibitors did not significantly promote the suppression of CO2 and CH4 emissions compared to a single application. Based on the structural equation model, soil NH4+-N and labile organic carbon were identified as key factors influencing the enrichment level of carbon cycle functional genes, as well as CO2 and CH4 emissions. This study suggests that soil labile organic carbon may influence the modulation of CO2 and CH4 releases by inhibitors and that multiple cycles of studies should be adopted when assessing the environmental impacts of inhibitors and alternative water irrigation.